1. Field of the Invention
The present invention relates to facilitating the exercise of control over aircraft and particularly rotary wing aircraft. More specifically, this invention is directed to means for automatically controlling the engagement and disengagement of the heading hold mode of an automatic flight control system. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
2. Description of the Prior Art
While not limited thereto in its utility, the present invention is particularly well suited for incorporation in an automatic flight control system for helicopters. When flying a helicopter a pilot must manipulate three separate control elements; i.e., the collective pitch stick, the cyclic pitch stick and the yaw pedals. Since a helicopter is an inherently unstable aircraft, manual flight control requires constant manipulation of these controls to maintain a predetermined attitude. Accordingly, to facilitate their use, automatic flight control systems for rotary wing aircraft have been devised. An early version of such an automatic flight control system is shown and described in U.S. Pat. No. 2,845,623.
Automatic flight control systems for helicopters include, in a yaw "channel", means for implementing a heading hold mode of operation. When in the heading hold mode the aircraft will fly "hands off" along a linear course selected by the pilot. A heading hold mode of operation, and the apparatus which permits such operation, are well known in the art and will be briefly discussed below in the description of the preferred embodiment of the present invention.
When the aircraft is in an automatic control mode, for example a heading hold mode, any deviations from the desired attitude will be sensed and appropriate corrective action automatically taken. In order to insure rapid and accurate aircraft response to a pilot input when the automatic flight control system is engaged, for example when an evasive maneuver is suddenly required, it is necessary that provision be made for disengagement of the automatic control or selected functional modes thereof. Such disengagement must be accomplished both automatically and quickly in order to insure that, when manual control is resumed, the automatic flight control system will not be resisting the input commands generated by the pilot. To this end, considering the heading hold mode of a helicopter automatic flight control system, sensor switches have customarily been provided on the yaw pedals. In order to assume manual control and disengage the heading hold mode, it was necessary for the pilot to place his feet on the yaw pedals thus actuating the pedal switches and thereby generating disengagement control signals. The employment of yaw pedal switches, however, required that the pilot have his feet on the pedals to make a turn and precluded the positioning of the pilot's feet on the yaw pedals during the heading hold mode. The necessity for the pilot to reposition his feet in order to shift between the manual and automatic control modes resulted, among other disadvantages, in a time delay incident to the resumption of manual control.
As a further requirement of an automatic flight control system for rotary wing aircraft, it is necessary that the circuitry which supervises the automatic engagement and disengagement of the heading hold mode be capable of distinguishing between a pilot input and a sudden comparatively large magnitude attitude change such as might be incident to a wind gust. Thus, the aforementioned disadvantages incident to the employment of yaw pedal mounted sensor switches can not be overcome merely by sensing the magnitude of an attitude change, such as a bank, since such attitude magnitude change sensing would be unable to discriminate between transient conditions, for which it is desired to compensate utilizing the heading hold logic, and those conditions where the pilot wishes to disengage the heading hold logic so as to execute a maneuver.
As a further complicating factor, aircraft, and particularly rotary wing aircraft, have a threshold speed of operation above which maneuvers such as turns should preferably be coordinated. As employed herein the term "coordinated turn" is synonymous with a bank turn, as opposed to a flat turn, and is a turn wherein the aircraft does not exhibit any lateral acceleration as characterizes a slip or skid. Thus, the yaw channel of the heading hold logic of an automatic flight control system should, above a preselected air speed, be capable of commanding and controlling automatic coordinated turns when in the manual mode; i.e., with the heading hold disengaged. In such a coordinated turn the yaw control will be automatically repositioned to produce the proper turn for the bank angle established by the pilot through manipulation of the cyclic pitch stick. Thus, should the pilot institute a maneuver, the heading hold mode should be automatically disengaged but sufficient control retained so as to produce a coordinated turn if the air speed is above a preselected level. Below this preselected air speed the disengagement of the heading hold mode should result in the pilot having total manual control.